Le chat reste immobile devant la fenêtre quand il regarde la pluie.

In French, singular countable nouns usually need an article much more often than in English.

So you get:

• le chat = the cat
• la fenêtre = the window
• la pluie = the rain

English sometimes leaves articles out in places where French does not, but French generally prefers to include them.

Rester means to stay or to remain. So reste immobile suggests that the cat remains motionless.

• Le chat est immobile = the cat is motionless
• Le chat reste immobile = the cat stays/remains motionless

Using rester adds the idea of continuity: the cat keeps still while watching the rain.

Here, immobile is not directly describing the noun inside the noun phrase. It is describing the subject after a verb of state.

In other words:

• Le chat est immobile
• Le chat reste immobile

This works like The cat is still or The cat remains motionless in English.

So immobile comes after the verb because it is functioning as a predicate adjective, not as an adjective placed before or after the noun inside le chat.

Yes, it does agree with chat. But in this case, the masculine singular form is simply immobile, so you do not see any extra ending.

With this adjective:

• masculine singular: immobile
• feminine singular: immobile
• masculine plural: immobiles
• feminine plural: immobiles

So even if the subject were feminine singular, the spelling would still be immobile.

Devant is a preposition meaning in front of or before.

So:

• devant la fenêtre = in front of the window

It does not need an extra de after it here.

This is different from:

• à la fenêtre = at the window / by the window
• près de la fenêtre = near the window

Devant la fenêtre specifically places the cat in front of the window.

Quand means when. It introduces a time clause:

• quand il regarde la pluie = when he watches the rain

In a present-tense sentence like this, quand can often feel like when or whenever, depending on context.

So the sentence could suggest either:

• a specific moment: the cat stays motionless when he is watching the rain
• a habitual idea: the cat stays motionless whenever he watches the rain

French usually uses the simple present where English may use either the simple present or the present continuous.

So il regarde can mean:

• he watches
• he is watching
• he looks at

French does have forms like être en train de, but they are used only when you really want to stress that an action is in progress.

Here, il regarde la pluie is the normal and natural way to say it.

Here la pluie means the rain. French commonly uses the definite article with nouns like this.

So:

• regarder la pluie = watch the rain

This is completely natural in French. It refers to the rain that is visible or present in the situation.

In this sentence, il refers to le chat.

So:

• Le chat ... quand il regarde la pluie
  = The cat ... when it/he watches the rain

French repeats the subject pronoun very regularly, even when the reference is obvious. Here there is no real ambiguity: il clearly points back to le chat.

A simple pronunciation guide would be:

Le chat reste immobile devant la fenêtre quand il regarde la pluie
luh shah rest ee-moh-beel duh-vahn lah fuh-netr kahn-teel ruh-gard lah ploo-ee

A few helpful notes:

• chat: the t is silent
• reste: the final e is silent
• fenêtre: the è sound is like eh
• quand il often has a liaison-like pronunciation: kahn-teel
• pluie is pronounced roughly ploo-ee, but very smoothly

Yes. The French present tense often covers both:

• what is happening now
• what usually happens

So this sentence can mean something like:

• The cat stays motionless when it watches the rain
• The cat stays motionless whenever it watches the rain

French leaves that distinction to context more often than English does.